1. Technical Field
The present invention relates generally to short arc discharge lamps and, more particularly, to a short arc discharge lamp in which a cathode electrode is provided with a tip part containing thorium oxide.
2. Description of the Related Art
Generally, short arc discharge lamps filled with xenon, which are used as light sources for projectors, or short arc discharge lamps filled with mercury, which are used as light sources of semiconductor or LCD exposure apparatuses, DC discharge lamps.
FIG. 3 illustrates a representative example of such short arc discharge lamps. A discharge lamp 1 includes an arc tube 2 which has a light emitting part 3 and sealing parts 4 formed on opposite ends of the light emitting part 3. A cathode electrode 5 and an anode electrode 6 are disposed opposite to each other in the light emitting part 3. The discharge lamp 1 is turned on by a DC lighting system.
In this way, the discharge lamp is turned on, and the spot of arc is fixed at the front end of the cathode electrode so that it can be used as a point light source. Therefore, when the discharge lamp is combined with an optical system, high light utilization efficiency can be realized.
Cathode electrodes which are typically used in such DC discharge lamps constantly function to emit electrons when the discharge lamps are turned on stationarily. Therefore, cathode electrodes made of high melting point metal mixed with an emitter material are mainly used so as to facilitate emission of electrons.
In such a discharge lamp which requires a point light source and high luminance, thorium oxide which can increase the operating temperature of the front end of the cathode electrode is generally used as the emitter material. However, because thorium oxide is a radioactive material, there are many regulations these days with regard to handling it. Hence, if there is no choice but to use thorium oxide for the cathode electrode, it is required to reduce thorium oxide content to the minimum.
In this respect, as a method of manufacturing a cathode electrode that contains thorium oxide as emitter material, a technique in which a body part of the cathode electrode is made of tungsten and a tip part made of thoriated tungsten containing thorium oxide is solid-phase bonded to a front end of the body part was introduced in Japanese Patent Laid-open Publication No. 2011-154927 (Patent document 1).
The structure of the cathode electrode according to this technique will be explained with reference to FIG. 4. The cathode electrode 5 includes a body part 51 which is disposed at a rear position, and a tip part 52 which is bonded to a front end of the body part 51. The body part 51 is made of pure tungsten, while the tip part 52 is made of thoriated tungsten which contains thorium oxide (ThO2) as an emitter material. In detail, thorium oxide content ranges from 0.5% to 3 wt %, for example, 2 wt %.
Overall, the cathode electrode 5 has a cylindrical shape, and its front end that includes the tip part 52 is tapered.
While the lamp is being turned on, the thorium oxide that is contained in the tip part 52 of the cathode electrode 5 is heated and thus reduced so that thorium atoms are obtained. Thorium atoms which are formed by the reduction process in the cathode electrode 5 are moved to the surface of the cathode electrode 5 mainly by grain boundary diffusion among tungsten crystal grains and are exposed to the outside. Thereafter, the exposed thorium atoms move to the front end of the cathode electrode and cover the front end of the cathode electrode. The covering layer of thorium atoms, lowering the work function of the cathode, promotes emission of electrons, thus improving electron emission characteristics.
However, thorium oxide, which contributes to improvement in electron emission characteristics, is limited to existing only at a very shallow depth from the surface of the front end of the cathode electrode.
The reason for this is as follows: Although thorium is required to be continuously supplied to the front end of the cathode electrode because thorium is evaporated and consumed from the surface of the front end of the cathode electrode, if the lamp is in the turned on state over a long time, the reduction of the thorium oxide slows down and eventually stops, whereby the supply of reduced thorium is not performed enough. Therefore, even when the cathode electrode contains a sufficient amount of thorium oxide therein, the surface of the cathode electrode may enter a thorium-exhausted state.
Such stagnation of reduction pertains to the following idea.
When reduction of thorium oxide occurs due to C (Carbon) which is present in the arc tube (through the carburization of the cathode electrode, etc.), CO (carbon monoxide) gas is generated. The reduction occurs on the surface of the tip part of the cathode electrode or in the interior of the tip part. If CO is generated and accumulated in the cathode electrode and the pressure in the cathode electrode is increased, it becomes difficult to induce the reduction of thorium oxide. As a result, it may be impossible to supply thorium atoms to the surface of the cathode electrode.
FIGS. 5A and 5B schematically show the sectional structure of the front end of the cathode electrode. FIGS. 5A and 5B respectively illustrate an initial lighting state and a thorium-exhausted state after a predetermined time has passed.
As shown in FIG. 5A, in the initial lighting stage, both the tip part 52 and the body part 51 are in a small crystal grain state.
After a predetermined lighting time has passed, as shown in FIG. 5B, although thorium oxide is in the tip part 52, tungsten crystal grains of the tip part 52 gradually coarsen compared to those in the initial lighting stage, because the tip part 52 is exposed to high-temperature heat by arc. Meanwhile, because the body part 51, which is lower in temperature than the tip part 52, has not been processed by doping, a recrystallization temperature of tungsten is lower than that of thoriated tungsten of the tip part 52, and tungsten crystal grains of the body part 51 also coarsen as time passes.
As such, with the passage of time, tungsten crystal grains of both the body part 51 and the tip part 52 coarsen.
In this state, grain boundaries among crystal grains decrease. The grain boundary decrease reduces the area of a portion which can occlude CO which is generated by reduction of thorium oxide in the tip part 52. Eventually, CO concentration increases, and reduction of thorium oxide is no longer conducted, whereby the supply of thorium is interrupted. Furthermore, even if the CO concentration in the body part 51 is comparatively low, crystal grains coarsen and decrease the area of the portion which can occlude CO. Thus, it becomes difficult for the body part 51 to occlude CO gas. As a result, CO gas is accumulated in the cathode electrode.
Thereby, CO pressure in the tip part 52 is increased, and reduction of thorium oxide in the tip part 52 is stagnated. Consequently, the surface of the cathode electrode enters a thorium-exhausted state.